1. Mechanical Testing

3. Here’s what we’re going to learn about!
Forces
Newton’s 2nd
Stress
Strain
Young’s Modulus s e

4. Force A force is a push or a pull
A Force is represented in diagrams by an arrow
SI unit of force is Newton (N)
N =
5 N
10 N
kg m
20 N s2

5. Newton’s 2nd Law of Motion
Force = (mass)(acceleration)
F = ma
Units
F: N = kg m/s2
m: kg
a: acceleration (due to gravity)
Symbol: a or g
Value: 9.81 m/s2

6. Newton’s 2nd Law of Motion
Force = (mass)(acceleration)
F = ma
Newton 2 can describe weight:
Weight = (mass)(acceleration)
W = mg
a = g = 9.81 m/s2
Only on Earth!

7. Example: Use Newton 2 to calculate your mass
Formulas
F = ma
1 lb = 4.45 N
This is a conversion factor
W= mg
150 lb
4.45 N =
668 N
Convert your weight in lb to SI unit of Newtons
1 lb
m= W/g
Rearrange the equation to solve for mass
m= 668 N / 9.81 m/s2
Plug and chug
m= 68 kg

8. Example: Use Newton 2 to calculate your weight on Mars!
Hey Hi
Formulas
F = ma
m = 68 kg
W= mg
1 lb = 4.45 N
W= mg
W= (68 kg)(3.71 m/s2)
W= 252 N
252N
1 lb
= 57 lb
a = g = 3.71 m/s2
On Mars
4.45 N

9. We just saw that my mass on Earth is 68 kg. What is my mass on Mars?
Question:
We just saw that my mass on Earth is 68 kg.
What is my mass on Mars?
Answer: 68 kg-my mass will not change.
My weight changes because acceleration changes on mars

11. Stress
Stress =
Force
Area
s = F A

12. Stress
Stress =
Force
Area
s = F A
12 N
1 m
Example
3 m

13. Stress
Stress =
Force
Area
s = F A
12 N
1 m
Example
3 m
F = 12 N

14. perpendicular to the force
Stress
Stress =
Force
Area
s = F A
12 N
1 m
Example
3 m
F = 12 N
A= (1m)(3m) = 3m2
We want the area that is
perpendicular to the force

15. Stress Example F = 12 N A= (1m)(3m) = 3m2 12 N = = 4 N/m2 3m2
Force
Area
s = F A
12 N
1 m
Example
3 m
F = 12 N
A= (1m)(3m) = 3m2
12 N
s = F A =
= 4 N/m2
3m2
= 4 Pascals
= 4 Pa

16. You try one!
3.14 N
s = F A
F =
Diameter = 2 m A=
s = F A =

17. Strain
Strain =
Lf - Li Li Li
e =
Lf - Li Li
initial

18. Strain
Strain =
Lf - Li Li Li
e =
Lf - Li Li
Force
initial Lf
final

19. Strain Lf - Li Strain = Li Lf - Li e = Li Li Lf - Li Force initial Lf
final

20. Example
e =
Lf - Li Li
1 mm
10 N
initial
Li =
5 mm
Lf =
10 N
final
e =

21. What if we make a plot of stress vs. strain?
Now put it together
What if we make a plot of stress vs. strain? s
Young’s Modulus e

22. We call this parameter Young’s Modulus
Stress vs. Strain
The slope of the stress-strain curve is a material property of stiffness
We call this parameter Young’s Modulus s
Young’s Modulus e

23. The higher the slope, the stiffer the material
Stress vs. Strain
The higher the slope, the stiffer the material
Material 2
Material 3 s
Material 1 e
Material 3 is the stiffest

24. H CH3 H C C C CH2 H Isoprene monomer
When mixing ratios of glue to borax, some students found that a small amount of polymer was made but there was a lot of left over water. This is because the vinyl alcohol can accept a limited amount of boron atoms before it is saturated. Once the vinyl alcohol chains are saturated with boron, a polymer is made, but the rest of the crosslinker is unable to bind to the glue. So, the excess borax solution remains in the cup.

25. H CH3 H H H CH3 H H C C C C C C C C H H H S S H H H C C C C C C C C H
Lab activity! H
CH3 H H H
CH3 H H C C C C C C C C H H H S S H H H C C C C C C C C H
CH3 H H H
CH3 H H